Heat the oil in a heavy skillet and cook the chicken breasts until nicely browned on one side, about 3 minutes over moderately high heat. Add the garlic cloves.
05Turn the chicken pieces and scatter the mushrooms over all. Continue cooking, shaking the skillet and redistributing the mushrooms so that they cook evenly. Cook about 3 minutes and add the balsamic vinegar and broth. Add the bay leaf and thyme.06Cover closely and cook over moderately low heat 10 minutes. Turn the pieces occasionally as they cook.07Transfer the pieces to a warm serving platter and cover with foil.08Let the sauce with the mushrooms cook, uncovered, over moderately high heat about 7 minutes. Swirl in the butter.09Remove and discard the bay leaf. Pour the mushrooms in sauce over the chicken and serve.